Test the fertility of your soil now

By Dairy News

EARLY SPRING is a good time to reassess the fertility of your soil and prepare silage paddocks, crop paddocks and determine how to maximise milker feed.

The best place to start is with a soil test, ideally in August to October or before soil moisture becomes limiting.

However, if you have historical soil tests, look to test at the same time of year as previously to ensure they are comparable.

Nutrient availability fluctuates throughout the year, therefore it is important to be consistent with timing of soil testing. Instead of testing every paddock, consider testing areas with similar soil type, topography and fertiliser/grazing history.

A soil test will indicate the most limiting nutrient of your soil with the key nutrients being phosphorus, potassium and sulphur.

It is difficult to accurately test for soil nitrogen as it is highly dependent on temperature and time between sampling and testing.

Instead, look for deficiencies in the plant such as yellowing on the tips of older leaves.

Phosphorus is vital for the establishment of new plants, root development, movement of energy in the form of adenosine triphosphate (ATP) and photosynthesis.

In Victoria, Olsen P is the test used to determine plant available phosphorus in the soil.

Research has shown that there is minimal response to phosphorus fertiliser when the Olsen P is above 27, therefore the ideal level is 20–27 for dairy farms.

Levels above pose a risk of losing nutrients into water systems and are unlikely to have a return on investment.

If your levels are considerably elevated, consider reducing phosphorus application levels for 6–12 months and continue to monitor levels with soil tests.

Potassium is one of the most important nutrients heading into spring, particularly for hay/silage paddocks.

On average hay and silage remove between 17 to 27 kg (more for silage, less for hay) of potassium per tonne of dry matter harvested.

Ensure potassium levels are adequate for silage growth and replenished after cutting, on top of maintenance application, to avoid a deficiency in the soil.

Be aware when feeding high potassium silage/hay to transitioning cows as this can impact calcium mobilisation and result in metabolic issues at calving.

If you are planning on feeding high potassium hay to transitioning cows, ensure you have other options available to balance the diet or apply potassium after cutting.

Ideally, look to top up your hay/silage paddocks with potassium throughout the year so a deficiency does not occur.

High levels of potassium can also interfere with the absorption of magnesium in the rumen leading to grass tetany.

This can be counteracted with the feeding out or application of additional magnesium.

When soil temperature drops below 8–10°C, it is often advisable to apply sulphate of ammonia (SOA) in conjunction with urea as a nitrogen source.

The two different forms of nitrogen increase the likelihood of uptake into the plant. SOA also applies sulphur which can become limiting over winter due to leaching and locking up in the soil.

Heading into spring, consider the soil temperature and if an SOA blend would be beneficial. As it warms up, urea will likely become the most economical option for nitrogen and the pasture will be in higher demand.

Economic rates of nitrogen are between 0 kg N/ha and 50 kg N/ha, and application beyond this will be less efficient.

Match your pasture growth rates to rate of nitrogen. In early spring, nitrogen response rates of perennial rye-grass are likely to be around 15–20 kg DM/kg of N applied.

Moisture must be available with application ideally being before a heavy rain event to reduce losses.

Pasture will only grow to the most limiting factor, so before increasing the nitrogen application rate, consider if there could be any other nutrients or soil components, including trace elements, pH and calcium to magnesium balance, reducing production.

• Ashley Burgess is a GippsDairy regional extension officer. This article was first published in GippsDairy’s How Now Gippy Cow.